Neural crest cell formation and migration is necessary for the normal development of cardiac septation. Neural crest ablation causes subsequent cardiac septation defects. Since homocysteine causes both neural crest- related midline closure defects and the neural crest-related cardiac defects, homocysteine appears to be preventing the normal migration and differentiation of neural crest cells. This hypothesis is consistent with preliminary data from in vitro experimental thus, the central hypothesis for this project is that homocysteine causes NGA conotruncal defects by altering the migration and/or differentiation of cardiac cells of neural crest origin. Based upon the following observations, we further hypothesize that the actions of homocysteine are mediated by an N- methyl-D-aspartate (NMDA) receptor. 1) NMDA receptor antagonists cause the same set of neural crest-related defects as homocysteine, 2) homocysteine is an NMDA receptor antagonist, 3) NMDA receptors play a key role in the Ca++-dependent migration of other cells during development, 4) neural crest cells display Ca++-dependent migration, and 5) neural crest cells express NMDA receptors. The following experiments are designed to test these hypotheses and to describe the key components of this system. 1. We will determine if homocysteine exposure alterations the formation, emigration, and/or migration of neural crest cells in stage 9-12 avian embryos. 2. Are homocysteine's actions on NC cells mimicked, or blocked, by NMDA receptor agents and by NMDA receptor subunit knockout? 3. We will identify the subtype of NMDA receptor that is expressed in NC cells and determine its ability to directly interact with homocysteine and to cause the developmental defects.